Here's a little exercise matrix that you can run through as prehab that will hit pretty much every articulation of the shoulder. Use a very light resistance band and make sure you are effecting the movement through voluntary contraction; exaggerate the eccentrics and isometrics.
I don't like the doorway stretch for my pec because I feel like it torques on my shoulders too much. I will do one of two variations.
#1 I lean up against a corner such that the corner is in pressing into the little crease where the pec crosses under the front delt. Once the distal attachment of the pec is locked in place by the corner, I rotate my torso away while simultaneously retracting the shoulder blade on the side with the pinned pec.
#2. I lie on a flat bench with a dumbbell in the hand of the side to be stretched. I perform the eccentric of a bent arm fly until I start to feel a stretch in the pec. At this point, I roll my torso away from the stretched side while simultaneously retracting the shoulder blade on the stretched side. I've found I can keep the dumbbell relatively close to my body (more elbow flexion) and obtain a pretty deep stretch in the pec without putting any strain on my shoulder.
I have also found a lat stretch that seems to help open up my shoulder flexion.
Find an object to hold on to (e.g. spotter pins on rack) that is around waist height or slightly lower. Bend over to grab the object such that your arm is straight and torso is about parallel with the floor; your knees should be slightly bent. Shift your weight back into the hips until you start to feel a stretch in the lat. Once you've achieved a stretch, laterally shift your torso in the direction of the side that is being stretched and laterally shift your hips AWAY from the side that is being stretched. This is essentially a pivot about the tangential line running through the shoulder on the side being stretched. This should produce a very deep stretch along the entire length of the lat.
In general, I do my stretches for 5 sets of 15 seconds or 3 sets of 25 seconds.
Since the shoulder has a number of articulations you'll really need to run through the combinations to see where there is noticeable weakness between the healthy and unhealthy sides. The articulations are:
1. flexion
2. extension
3. abduction
4. adduction
5. internal rotation
6. external rotation
Internal and external rotation can be executed in combination with 1-4.
I have found that NSAIDs seem to be most effective when taken before bedtime. I have also found that ibuprofen seems to work better for me than naproxen sodium.
Contrast showers provide some short term relief.
If this doesn't clear up within a few weeks I would definitely go through the steps of seeing an ortho and starting PT. Your insurance company will probably want to see 4-6 weeks of conservative treatment before agreeing to an MRI.